A circle with radius $8$ has a sector with a $\dfrac{5}{4}\pi$ radian central angle. What is the area of the sector? ${64\pi}$ $\color{#9D38BD}{\dfrac{5}{4}\pi}$ ${40\pi}$ ${8}$
First, calculate the area of the whole circle. Then the area of the sector is some fraction of the whole circle's area. $A_c = \pi r^2$ $A_c = \pi (8)^2$ $A_c = 64\pi$ The ratio between the sector's central angle $\theta$ and $2 \pi$ radians is equal to the ratio between the sector's area, $A_s$ , and the whole circle's area, $A_c$ $\dfrac{\theta}{2 \pi} = \dfrac{A_s}{A_c}$ $\dfrac{5}{4}\pi \div 2 \pi = \dfrac{A_s}{64\pi}$ $\dfrac{5}{8} = \dfrac{A_s}{64\pi}$ $\dfrac{5}{8} \times 64\pi = A_s$ $40\pi = A_s$